So, first, thank you all for the advice and the welcome.
I took the plunge this morning, edited my long post into a letter, read it over several times and then handed it to my wife: "I want you to read this; it's hard for me to talk about, so I wrote it down."
She sat on the bed and I took a seat and patiently (but inside anxiously) waited. When she was done I asked if she wanted to talk or wanted to take some time. She started slowly asking questions.
Crash and burn. I won't recount the entire discussion because it meandered a lot, but by and large not a good reception. Here are some highlights
1) We've had a cogent discussion as two adults about our relationship. This is very good.
2) I've hurt her feelings, which is bad. I didn't intend to, but I completely understand. She's interpreted my desire to pursue relationships with other women as "Something is wrong (with me?). My husband doesn't love me; I'm not providing enough XYZ for him."
I tried to address this as directly as possible; I love her. A lot. More than anything. It's not about her not giving me something, it's about the possibility top bring more love into our life.
3) She does not not not understand polyamory no matter how many ways I try to describe it. She's barely receptive to reading more about it. She put it into internet terms, "I don't understand how you can have so much bandwidth." And this, I think, is the crux of the disconnect: she is a hardcore introvert. Relationships don't fuel her; quite the opposite, they drain her. She keeps her rapports to a minimum, friendship, family and otherwise. And with the possibility of a child, that's another relationship, which, in her mind, is another resource drain. Good relationships for her are about comfort and stability rather than energy and possibilities.
(In hindsight, I feel that should have exercised my empathy a little more to foresee this. I'm quite aware of my wife's introversion; it seems silly to think that she would see polyamorous romantic relationships any differently.)
There was lots of crying involved. :-( I feel a bizarre mixture of guilt and relief. (I hate hurting anyone's feelings, especially my wife's; but we finally had a good discussion, even if it was hard and a little painful). I did my best to reassure her that I love her, that she is the priority, that I wanted to talk and not issue ultimatums.
This is going to take some time. I think I've dropped a really big rock in the pond and the water may be choppy for a while.